Orthostatic intolerance, also known as the postural tachycardia syndrome (POTS), is the most common cause of chronic fatigue syndrome (CFS) in the young. Our lab has demonstrated that many CFS/POTS patients have a redistributive form of central hypovolemia that is associated with splanchnic hyperemia and hypervolemia also known as splanchnic pooling. Furthermore, cutaneous studies have provided separate evidence for nitric oxide (NO) excess although it remains unclear which nitric oxide synthase (NOS) isoforms are involved. We hypothesize that CFS/POTS is caused by either increased neuronal NOS activity (nNOS, NOS-1) or increased inducible NOS (INOS, NOS-2), that there is evidence for increased NOS expression at tissue level, and that inhibition of NO-dependent vasodilation can reduce splanchnic hyperemia and improve orthostatic tolerance. To explore these hypotheses, 20 CFS/POTS patients, 20 CFS patients without POTS (CFS/~POTS), and 20 control subjects will be studied in order to accomplish the following Specific Aims: 1) We will determine the isoform dependence of increased cutaneous NO in CFS/POTS by measuring NO-dependent local heating and acetylcholine cutaneous blood flow responses combined with isoform selective NOS inhibitors. 2) We will determine whether protein and mRNA expression of NOS isoforms are increased in white blood cells and skin biopsies of CFS/POTS, compared to CFS/~POTS, and controls using Western blot and RT- PCR techniques. 3) We will test the hypothesis that the guanylate cyclase inhibitor methylene blue versus phenylephrine differentially decreases splanchnic pooling in CFS/POTS patients compared to healthy control subjects. Relevance: Chronic orthostatic intolerance, or the inability to remain upright, affects a large number of Americans, making it impossible for these individuals to study or maintain jobs. This condition is also known as the postural tachycardia syndrome and is the main cause of chronic fatigue syndrome in the young. We have evidence that many POTS patients have excessive pooling of blood in the gut circulation when upright that is related to a small yet potent fundamental molecule called nitric oxide (NO). In the proposed studies, we will demonstrate the origin of the excess in NO, and while using an already approved medication, we will test whether we can improve our patients so that they can effectively return to work and school.